1. Field of the Invention
The present invention relates to an ink jet printing apparatus and an ink jet printing method for printing an image by causing an ink ejection print head to scan in a direction crossing a print medium transport direction.
2. Description of the Related Art
As information processing devices such as personal computers proliferate in recent years, printing apparatus as an image forming terminal have also made rapid advance and come into widespread use. Of various printing apparatus, an ink jet printing apparatus is characterized by a construction in which ink is ejected from printing elements or nozzles onto a print medium, such as paper, cloth, plastic sheet and OHP sheet, to form an image on it. Since it employs a non-impact type printing system, the ink jet printing apparatus has many excellent features including an ability to perform the printing operation at low noise, at high resolution and at high speed, an ability to deal with a color printing with ease, and a low cost. Because of these advantages, the ink jet printing apparatus is now established as a mainstream printing apparatus for personal use.
The advance of the ink jet printing technology has been making improvements toward higher printed image quality, faster printing speed and lower image forming cost. Combined with proliferation of personal computers and digital cameras (including the ones that as single devices perform their functions and the ones that are integrated into other devices such as mobile phones), the progress in ink jet printing technology has made great contributions to the prevalence of the printing apparatus even among personal users. With the widespread use of the printing apparatus, there are growing calls also from personal users for further improvement of image quality. Particularly in recent years, users are demanding that photographs be able to be printed easily at home and that the printed images have a quality that matches a silver salt picture.
When compared with common silver salt pictures, images formed by the ink jet printing apparatus have a long-standing problem of graininess characteristic of the ink jet printing. A variety of measures to reduce this graininess have been proposed in recent years and there are many printing apparatus incorporating such measures. For example, there is an ink jet printing apparatus with an ink system which, in addition to the ordinary cyan, magenta, yellow and black inks, has light cyan and light magenta inks with lighter densities than the ordinary inks. In this ink system the use of light cyan and light magenta in areas with low densities can reduce graininess. In areas with high densities, using the ordinary cyan and magenta for printing can realize a wider color reproduction and smooth tonality.
Another proposed method involves reducing the size of dots formed on a print medium to reduce the graininess. For this purpose, a research is under way to reduce a volume of each ink droplet ejected from nozzles arrayed in a print head. In this case, not only reducing the volume of ink droplets but arranging a greater number of nozzles at smaller intervals can produce a high resolution image without slowing down the printing speed.
While the ink jet printing apparatus for personal use is required to print high quality images that match the quality of silver salt pictures, there are not a few occasions where ordinary documents including texts and tables are output. In printing documents, a greater importance is placed on a fast printing speed than on a high image quality that equals the silver salt pictures. Therefore, general ink jet printing apparatus are provided with a plurality of print modes so that the user can choose a desired print mode according to the kind of data being printed.
These ink jet printing apparatus have progressed in speed and image quality to meet the demands of the times and, as digital cameras become more widespread, the amount of data to be handled has also increased. One print image may have varying dot density distributions, including high duty areas and low duty areas, requiring varying amount of electricity and print data to drive a print head. To allow for a constant printing performance regardless of such electric power variations, circuits have to withstand a large capacity power supply, resulting in a printing apparatus itself becoming large in size and expensive.
In ordinary printing, there are very few pages of image that require constantly activating the largest number of nozzles arrayed in the print head. Even in one page of image, the area that is printed using the nozzles to the full capacity usually occupies only a part of it. To realize a low-cost, small printing apparatus, it is proposed to control a printing method according to the number of printed dots in a predetermined range. That is, the proposed method involves limiting the amount of electricity used in a predetermined length of time to assure a certain level of print quality even at locations where the image is printed at high duty while maintaining a high printing speed.
In Japanese Patent Publication No. 62-41114, for example, a printing apparatus is disclosed which counts a total number of dots and, when a predetermined value is exceeded, selects an appropriate printing speed to enable the printing of the largest number of dots.
Japanese Patent Application Laid-open No. 06-47290 discloses a method which, when a predetermined dot duty is exceeded in an area, prints that area in a plurality of divided passes.
Japanese Patent Application Laid-open No. 09-226185 discloses a method which divides a line buffer memory into segments and, when a print duty in any segment area exceeds a predetermined threshold, makes a decision as to whether a two-way printing or one-way printing is to be performed.
Japanese Patent Application Laid-open No. 09-226175 discloses a method which, as in Japanese Patent Application Laid-open No. 09-226185, checks if the print duty exceeds a predetermined value and, based on the check result, performs a dot-thinning printing.
Japanese Patent Application Laid-open No. 10-217436 discloses a method which introduces a divided control printing system capable of dividing a print area by considering characteristics of the nozzles and of utilizing not only a peak power but also a power supply as a whole at a maximum possible efficiency.
Further, Japanese Patent Application Laid-open No. 05-318770 discloses a method which changes a printing mode during the divided printing according to the kind of paper.
Further, Japanese Patent Application Laid-open No. 07-285227 discloses a method which, when a remaining volume of ink is small during the printing operation, checks image data to change the printing mode to an appropriate one.
However, the conventional ink jet printing apparatus described above may experience the following problems.
In the ink jet printing apparatus disclosed in Japanese Patent Publication No. 62-41114, there are occasions where a speed of a carriage mounting the print head and a drive frequency of the print head are lowered during the printing operation according to the number of dots to be formed. In that case, since in the ink jet system a dot landing position accuracy and the volume of ink ejected for one dot are influenced by the carriage speed and the drive frequency, an area printed at low speed and an area printed at normal speed have different densities or gradation levels, resulting in density variations on a printed image. Such density variations, though they may be adjusted by an ejection volume control, will lead to a complex construction of the printing apparatus. Thus, the technique disclosed in this publication is difficult to apply to low-cost printers.
In the ink jet printing apparatus disclosed in Japanese Patent Publication No. 06-47290, when a print medium is used which exhibits relatively sharp density changes in dot overlapping portions, harmful effects, such as density variations and seam lines, are produced on the printed image depending on how ink dots overlap one another.
If the printing mode is limited to the two-way or one-way printing as in Japanese Patent Application Laid-open No. 09-226185, the appearance of color may change from one ink application order to another depending on the print medium used, degrading an image quality.
In Japanese Patent Application Laid-open No. 09-226175, since the printing is done by thinning print data, an originally intended image cannot be formed.
Japanese Patent Application Laid-open No. 10-217436 discloses a method that introduces a divided control printing system capable of dividing a print area by considering characteristics of the nozzles and of utilizing not only a peak power but also a power supply as a whole at a maximum possible efficiency. However, if a high speed printing is performed by using a recently introduced print head made up of 512 small nozzles each ejecting fine droplets of 5 pl or less and arrayed at narrow pitches of 1200 dpi or less, the following problem will surface. A so-called end nozzle dot deflection becomes notable, a phenomenon in which the ink droplets ejected from the nozzles at the ends of the print head are deflected by an air flow and land on positions deviated toward a central portion of the head. A simple divided printing alone, however, cannot prevent this end nozzle dot deflection from producing white lines significantly degrading the image quality.
Japanese Patent Application Laid-open No. 05-318770 discloses a method which performs a divided printing by limiting the print head scan direction to only one direction for a special print medium and, for plain paper, performs the divided printing in two directions. However, the inventors of this invention have found that if the divided printing is performed on the special print medium by thinning print data, only a band area that was subjected to the divided printing has a different density, a so-called band variation phenomenon. It is therefore not preferable to apply the divided printing described in the above reference to the printing of picture image that places greater importance on the tonality.
Further, Japanese Patent Application Laid-open No. 07-285227 discloses a method which, when an ink volume remaining in an ink tank is running low during the printing operation, checks if the data to be printed is characters or image and, if it is decided to be characters, allows the printing operation to be continued to the end. However, this reference does not give any descriptions on the relation between the ink remaining volume or ink flow volume and the divided printing method.